1. Field of the Invention:
This invention relates to interrupters for use in controlling the visual and audible signals supplied to key telephone sets for use in key telephone systems.
2. Description of the Prior Art:
Key telephone systems provide a group of key telephone sets with access to two or more telephone lines so that an incoming call may be answered or an outgoing call placed on any one of the sets. Also, a call may be held until the appropriate party can answer the same.
Each key telephone set includes a line selection button associated with each line to enable the user to connect the handset (transmitter-receiver) to a selected line. When a call is received on one of the lines a small lamp located beneath the associated line selection button is energized with a signal having a preselected duty cycle and frequency, e.g., 50% on, 50% off at 1 Hz, commonly called a flash signal. An electromechanical ringer in the telephone set is also energized with a signal having another duty cycle and frequency, e.g., two seconds on and four seconds off. Such visual and audible signals apprise the key telephone set users of the incoming call and identify the particular line over which the call is being received.
When a call is answered at one of the key telephone sets the line selection lamp associated with that line is energized with a steady signal to apprise all of the users that that particular line is in the seize or talk condition. When a line is placed in the hold condition by actuation of a hold button at one of the key telephone sets the associated line selection lamp is energized with a so-called winking signal, e.g., a signal having an on duty cycle of seventy-five percent (75%) and a frequency of about 2 Hz.
A source of ten volt 50-60 Hz is normally supplied at the location of the key telephone sets for providing the energy to the line selection lamps. A ringing signal generator having a 110 to 120 volt output at 20-30 Hz is also normally provided at the location of the key telephone sets for furnishing power to the electromechanical ringers in the sets. A line card circuit such as described in U.S. Pat. No. 3,647,983 is connected to each incoming line and to each of the key telephone sets for providing the necessary supervisory and control functions. For example, such line cards include switches for connecting the line selection lamps to the appropriate source of signals, that is, wink, flash and steady signals, and a switch is also provided in the line cards for connecting the source of ringing power to the electro-mechanical ringers in the telephone sets.
An interrupter is connected between the power sources, e.g., 10 volts at 60 Hz and 110 volts at 30 Hz, and the line cards to interrupt the circuit to the power sources in predetermined patterns to provide the lamp wink, lamp flash and ringing signals required from the two separate power sources.
The conventional interrupter employed in the past includes a motor driven cam arrangement for opening and closing sets of contacts. Such electromechanical interrupters may have a limited lifetime due to the wear of the motor, spring, cam and contact surfaces. Furthermore, the current carrying capacity of prior art electromechanical interrupters is limited by the contact rating of the device and therefor more than one interrupter may be required in a key telephone system which employs a large number of telephone sets.
An electronic interrupter has been proposed in U.S. Pat. No. 3,059,061 to S. L. Merkel for establishing several predetermined interruption patterns between a ringing signal source and the telephone sets. This circuit employs transformers for providing electrical isolation between the interrupter circuit and the electromechanical ringers. This interrupter is expensive, complex and bulky due to the large number of components employed and the use of transformers for providing electrical isolation. Furthermore, this interrupter does not control the energy to the line selection lamp to provide the flash and wink signals.
U.S. Pat. No. 3,647,948 to Morris Ribner, et al. discloses a complex solid state circuit for generating telephone supervisory tone and ringing signals for use in electronic switching systems.
A need has long existed for a compact, low cost, reliable interrupter capable of handling high current loads.